This page is intended for college, high school, or middle school students.
For younger students, a simpler explanation of the information on this page is
available on the
Kid's Page.

Lift is the force that
directly opposes the weight
of an airplane and holds the airplane
in the air. Lift is generated by every part of the airplane, but
most of the lift on a normal airliner is generated by the
wings.
Lift is a mechanical
aerodynamic force produced by the motion
of the airplane through the air.
Because lift is a force, it is a
vector quantity,
having both a magnitude and a direction associated with it.
Lift acts through the
center of pressure
of the object and is directed perpendicular to the flow direction.
There are several
factors
which affect the magnitude of lift.

HOW IS LIFT GENERATED?

There are many explanations for the generation of lift found in
encyclopedias, in basic physics textbooks, and on Web sites.
Unfortunately, many of the explanations are misleading and incorrect.
Theories on the generation of lift have become a source of great
controversy and a topic for heated arguments.
To help you understand lift and its origins, a series of pages will
describe the various theories and how some of the popular theories
fail.

Lift occurs when a moving flow of gas is turned
by a solid object. The flow is turned in one direction, and the lift
is generated in the opposite direction, according to Newton's
Third Law of action and reaction. Because air is a gas
and the molecules are free to move about, any solid surface can
deflect a flow. For an aircraft wing, both the
upper and lower surfaces contribute to the flow turning. Neglecting
the upper surface's part in turning the flow leads to an
incorrect theory
of lift.

NO FLUID, NO LIFT

Lift is a mechanical force. It is generated by the interaction and
contact of a solid body with a fluid (liquid or gas). It is not
generated by a force field, in the sense of a gravitational
field,or an electromagnetic field, where one object can
affect another object without being in physical contact. For lift to
be generated, the solid body must be in contact with the fluid: no
fluid, no lift. The Space Shuttle does not stay in space because
of lift from its wings but because of orbital mechanics related to
its speed. Space is nearly a vacuum. Without air, there is no lift
generated by the wings.

NO MOTION, NO LIFT

Lift is generated by the
difference in velocity
between the solid
object and the fluid. There must be motion between the object and the
fluid: no motion, no lift. It makes no difference whether the object
moves through a static fluid, or the fluid moves past a static solid
object. Lift acts perpendicular to the motion. Drag
acts in the direction opposed to the motion.

You can learn more about the
factors
that affect lift at this web site.
There are many small interactive programs here to let you explore the
generation
of lift.

You can view a short
movie
of "Orville and Wilbur Wright" discussing the lift force
and how it affected the flight of their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.